Muscle building requires a positive energy balance - consuming more calories than you expend. Your body uses this caloric surplus, combined with the training stimulus, to synthesise new muscle tissue. Without a surplus, muscle gain is severely limited or impossible for most people. The question is: how large should that surplus be?
A modest surplus of 200-300 calories above your Total Daily Energy Expenditure (TDEE) is optimal for most natural athletes wanting to maximise muscle gain while minimising fat accumulation. This "lean bulk" approach produces gradual, sustainable muscle development. More aggressive surpluses (500+ calories over TDEE) accelerate fat gain without proportionally increasing muscle gain rates, since muscle protein synthesis has a biological ceiling that additional calories cannot override.
Calculate your TDEE using an online calculator (input age, weight, height, sex, and activity level). Add 200-300 calories. Weigh yourself weekly under consistent conditions (same time of day, same state of dress). If you're gaining more than 0.5-1kg per week (which would be almost entirely fat), reduce calories slightly. If you're gaining 0.2-0.4kg per week, you're in the lean gaining zone. Track progress over months, not weeks - natural muscle building is slow. A natural male lifter can realistically gain 1-2kg of muscle per month in the beginner phase, slowing to 0.5-1kg per month as an intermediate and less than 0.5kg per month as advanced. Patience and consistency with a modest surplus is the optimal long-term approach.